Confirmation of the existence of the X17 particle

In a 2016 paper, an anomaly in the internal pair creation on the $M1$ transition depopulating the 18.15 MeV isoscalar 1$^+$ state on $^8$Be was observed. This could be explained by the creation and subsequent decay of a new boson, with mass $m_Xc^2$ = 16.70 MeV. Further experiments of the same transition with an improved and independent setup were performed, which constrained the mass of the X17 boson ($m_Xc^2$) and its branching ratio relative to the $\gamma$-decay of the $^8$Be excited state ($B_X$), to $m_Xc^2$ = 17.01(16) MeV and $B_X$ = 6(1) 10$^{-6}$, respectively. Using the latter setup, the $e^+e^-$ pairs depopulating the 21 MeV $J^\pi$ = 0$^-$ $\rightarrow$ 0$^+$ transition in $^4$He were investigated and a resonance in the angular correlation of the pairs was observed, which could be explained by the same X17 particle, with mass $m_Xc^2$ = 16.98 ± 0.16(stat) ± 0.20(syst) MeV.